This is an improvement to the invention of my U.S. Pat. No. 4,763,419, issued Aug. 16, 1988, entitled "Optical Viewing System", the entire disclosure of which is hereby expressly incorporated by reference.
The present invention relates generally to optical sighting instruments such as telescopic instruments incorporating a compass for simultaneously viewing and determining the direction of an object and, more particularly, to such instruments which eliminate parallax and further which automatically compensate for magnetic declination or variation such that directions referenced to true north may be determined employing a compass which seeks magnetic north.
As is known, parallax occurs in reading instruments such as compasses because a compass needle, for example, does not lie directly against its scale. Unless the observer's eye is aligned correctly with the needle and the scale, an apparent reading will occur which is to one side or the other of the actual reading.
In the instrument of my U.S. Pat. No. 4,763,419, a beamsplitter is employed for combining or superimposing an image of a moving compass scale with an image of a distant object viewed through the instrument, such that the two images may be viewed simultaneously and the direction of a sighted object determined while viewing through the instrument.
In most geographical locations, the direction of magnetic north does not coincide with the direction of true north. Stated alternatively, there is an angle between the local magnetic meridian and the geographic meridian, which is known as magnetic declination or magnetic variation. Maps and charts are available which indicate this phenomenon as easterly or westerly magnetic declination or variation. Magnetic declination is expressed in degrees. To convert or correct a magnetic compass reading to a reading referenced to true north rather than magnetic north, the local magnetic declination is either added or subtracted from the magnetic compass reading.
To facilitate correction for magnetic declination, the instrument of my U.S. Pat. No. 4,763,419 includes a movable sighting indicia which may be moved to the left or the right in the field of view through the instrument to adjust for local magnetic declination. The beamsplitter optics also combines or superimposes an image of the movable sighting reference with images of the compass scale and the object so that all three are viewed simultaneously. Such use of a beamsplitter to superimpose images effectively eliminates parallax. In the use of the instrument of my U.S. Pat. No. 4,763,419, a distant object being viewed or sighted is aligned with the movable sighting reference or cross hair, and the compass reading in degrees is read where the image of the cross hair intersects the compass scale, as is illustrated in FIG. 4 of U.S. Pat. No. 4,763,419.
Although it is an effective and useful instrument, the instrument of my U.S. Pat. No. 4,763,419 is subject to improvement. For example, it would be desirable to provide a means for compensating for magnetic declination, while allowing a cross hair or other indicia used to sight a distant object to be centered within the field of view at all times. It would also be desirable to provide an instrument suitable for night use. There are other areas of improvement, as will be apparent from the description hereinbelow.